This invention relates generally to gas-insulated equipment and more particularly to a gas-insulated transmission line having a closed particle trap.
Compressed gas-insulated tramsmission lines are being used in an ever increasing scale in recent years due to the desirability of increasing safety, problem in acquiring the necessary right-of-way for overhead lines, and higher power loads required by growing metropolitan areas and growing demands for electrical energy. Gas-insulated transmission lines typically comprise a hollow sheath, a conductor disposed within the sheath, a plurality of solid insulating spacers which support the conductor, and a compressed gas such as sulfur hexafluoride in the sheath to electrically insulate the conductor from the sheath. The typical assembly has been fabricated from relatively short sections of hollow ducts in which conductors and insulators are inserted. This assembly is usually completed in the factory, and the sections are welded or otherwise secured together in the field to form the transmission line. Gas barriers may be provided at intervals along the length of the assembly and, after evacuation of the line, an insulating gas is forced into the sheath under pressure. It is also known to provide a particle trap in gas-insulated transmission lines as it is disclosed in the patent to Trump, U.S. Pat. No. 3,515,939.
Trump disclosed a means for deactivating and eliminating the deleterious effects of any conducting or semiconducting particles which may be present within the gas insulated transmission line. In the patent, Trump describes the use of electrodes placed inside the outer sheath to create low field regions which trap and deactivate the particles. Whenever a particle enters the low field region, it cannot acquire enough force to propel it out of the region and thus is trapped in the low field region so that it cannot initiate breakdown of the line. However, it has been found that under certain circumstances the field reducing means of Trump do not function as effectively as may be desired. For example, if the transmission line is not horizontal but instead is sloped, the particles which had been trapped in the low field region can be moved by mechanical vibration or gas flow out of the low field region. Whenever these particles are outside the low field region, they can travel between the inner and outer conductors and so initiate breakdown.